🚸(frontend) auto-populate prejoin screen with user's default full name

Streamline user joining a room by pre-fillig name field from
their ProConnect data. Reduces friction and typos in the join flow.

This feature will be mostly used by new users. Recurrent users have
their previous choices persisted in local storage.

src/frontend/src/features/rooms/components/Join.tsx

@@ -2,6 +2,7 @@ import { useTranslation } from 'react-i18next'
 import { PreJoin, type LocalUserChoices } from '@livekit/components-react'
 import { Screen } from '@/layout/Screen'
 import { CenteredContent } from '@/layout/CenteredContent'
+import { useUser } from '@/features/auth'
 
 export const Join = ({
   onSubmit,
@@ -9,6 +10,7 @@ export const Join = ({
   onSubmit: (choices: LocalUserChoices) => void
 }) => {
   const { t } = useTranslation('rooms')
+  const { user } = useUser()
 
   return (
     <Screen layout="centered">
@@ -20,6 +22,7 @@ export const Join = ({
           camLabel={t('join.videoinput.label')}
           joinLabel={t('join.joinLabel')}
           userLabel={t('join.usernameLabel')}
+          defaults={{ username: user?.full_name }}
         />
       </CenteredContent>
     </Screen>